Yes, that is the case. Occasionally a major title like His Girl Friday or McLintock! will slip thru the cracks. Many p.d. titles are "b" movies from past decades, foreign films, indies who didn't have the money to do a copyright in the first place, etc.

Do a Google search for "public domain films." I had an account do that and found lots of info on the subject.

-Films older than 1923 are most likely in public domain (be careful that they have not been revised or added to, such as the addition of a soundtrack to a silent film)

-Films from 1923 to 1963 whose copyright owners failed to extend their copyright during the 28th year (the last year of their original term) are most likely in the public domain. If extended they have protection for a total of 95 years from the time they were published. Revisions or additions to works from any era can add additional copyright protection. Example would be colorizing a black and white film.

-Also films may be protected by copyrights on books, plays, or other works from whence the story originated.

-Films made by or for the U.S. government are most likely public domain.

-Films made before 1989 that didn't include a proper copyright notice are normally public domain. Films made after March 1989 don't require a proper copyright notice to be protected, so almost nothing after that time is public domain except government works.

-Treaties like GATT and NAFTA caused foreign films that used to be in the U.S. public domain to now become protected. So, unless the film was made before 1923, a foreign film is now most likely copyright protected.

One of the most interesting statements on the referenced openflix page is "Copyright status for any particular movie can only be determined though the judicial system." They say they have made a good faith effort to determine what is in the public domain, but...they still make this statement to cover their backsides. Almost every source you can find for public domain recommends you do your own research. Even the Library of Congress Copyright Clearance Office who offers to do your research for you for a fee will not absolutely guarantee that a work found by them to be public domain is actually and truly without possible legal claims for copyright. It's a complicated issue.

That said, I tend to think that if an older classic film is being reproduced, broadcast, and sold without royalties being paid to the previous copyright holder or the creator, then it would probably be fairly safe to play it in a theater without fear of reprisal. However, that is just my opinion. Your mileage may vary. I wouldn't want to be the first to test the waters, but if I see that there are several major video companies selling copies of a work all over the internet and they don't have a licensing agreement, then I might be willing to take a chance. How would I know they don't have a licensing agreement? That is where my own research becomes necessary. I would check websites that list public domain works to see if the particular film is on their lists, hopefully on several lists. Certain companies seem to deal almost exclusively in public domain works. I would also check the library of congress website. I would never trust a single source.

As for which exact sites provide lists, I will let you do your own web search. It's not that hard and I don't want to promote any specific one for fear they may have made a mistake here or there.

Also, be careful that things like musical scores don't have their own copyright protection. Seems to me that was how "It's a Wonderful Life" was removed from the public domain and placed back under copyright protection. The music was copyrighted.